How you will use it (modify, resize, add text, etc.); and
Where you'll be using it (in a presentation in class, on Blackboard, as a handout, etc.) because this will affect your selection process.
Ask yourself if the image is integral to the work - in which case you can apply fair dealing to the work as a whole OR whether the image is standalone - meaning you then have to track down the original source and determine whether you can/should use the image in the way that you want.
Then apply the 4 Steps - Refer to the handout you received at the workshop.
Things to Keep in Mind
There is a big difference between posting to a password protected website such as Blackboard and posting to a publicly accessible site such as a blog on WordPress. As a rule, only post images to open sites where you have been given permission or a license (such as creative commons) to use it in the first place.
If using an image under one of your educational rights, make sure there's an educational reason to use the image in the first place. "Wow" factor or decoration don't count in most cases.
Never use an image at all if you have reason to suspect it was not legally created or uploaded. Ask yourself if the poster had the authority to reproduce and distribute the image.